Circuit board and method for manufacturing the same

ABSTRACT

A circuit board includes an electrical insulator layer formed of a reinforcer sheet with density distribution in its in-plane direction, an electrical conductor filled in a plurality of inner via holes provided in the electrical insulator layer in its thickness direction, and a wiring layer connected to the electrical conductor. The inner via holes provided in a high-density portion of the reinforcer sheet are formed to have a smaller cross-section than the inner via holes provided in a low-density portion of the reinforcer sheet. In this manner, it is possible to provide a circuit board that can achieve a high-density wiring and an inner via connection resistance with less variation, when a base material including a reinforcer sheet with density distribution in its in-plane direction such as a glass-epoxy base material is used for an insulator layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a circuit board and a method formanufacturing the same. In particular, the present invention relates toa circuit board using a reinforcer sheet with density variations in itsin-plane direction and a method for manufacturing the same.

2. Description of Related Art

In recent years, accompanying the reduction in size and weight and theimprovement in function and performance of electronic equipment, therehas been an increasing demand for low-cost multilayered circuit boardsallowing a high density mounting of semiconductor chips such aslarge-scale integrated circuits (LSIs), in the field of not onlyindustrial appliances but also home electronic appliances.

In order to respond to such a market demand, the technology has beendeveloped in which, instead of a conventional ceramic multilayeredboard, a resin multilayered circuit board that can be supplied at lowercost is made suitable for the high density mounting (a high densitywiring board).

An example of such a circuit board includes a resin multilayered boardhaving an inner-via-hole structure over all the layers as disclosed inJP 6(1994)-268345 A. This resin multilayered board adopts an inner viaconnection method that can connect desired positions of desired wiringlayers by an electrically conductive paste, namely, the inner-via-holestructure over all the layers, thereby providing a low cost circuitboard suitable for the high density mounting.

In a method for manufacturing this circuit board, inner via holes firstare formed in a compressible insulator layer (an aramid-epoxy prepreg),and an electrically conductive paste is filled in the through holes.Thereafter, copper foils are superposed on both sides of the insulatorlayer, followed by heating and compression with a hot press, therebycuring resins in the insulator layer and the electrically conductivepaste. This adheres the copper foils to the insulator layer andelectrically connects the copper foils on both sides via theelectrically conductive paste. Finally, the copper foils on both sidesare processed into a wiring pattern, thus completing a double-sidedcircuit board.

Because of its high-density wiring and low connection resistance withless variation, this circuit board is highly valued in the market.

The reason why the high-density wiring is needed has been describedabove, while the usefulness of the connection resistance with lessvariation will be described in the following. That is, circuitresistance including the connection resistance is an important parameterfor a circuit design. Accordingly, if the circuit resistance varies fromone product to another, this causes a problem in that the circuit designis impossible or that the circuit resistance of the product deviatesfrom a designed value so that the product cannot operate properly. Thus,the connection resistance has to have less variation.

Especially, more inner via holes are involved in one circuit in theconnection by the inner via holes than in the conventional connection bythe through holes. Therefore, there is a more stringent requirement withrespect to variations.

However, the above-described circuit board technology having theinner-via-hole structure over all the layers has had a followingproblem. As the insulator layer mentioned above, a composite material ofan aramid non-woven fabric as a reinforcer and an epoxy resin (anaramid-epoxy base material) is used. In this case, because of their highmoisture absorbency, it is necessary to manage aramid fibers so as toprevent them from absorbing moisture by vacuum-packing or the like. Suchmanagement would increase cost.

On the other hand, a glass-epoxy base material used in a general circuitboard is a material obtained by impregnating woven fabric made of glassfibers with an epoxy resin. Since the glass fibers do not absorbmoisture, using the glass-epoxy base material is advantageous in themanagement of water absorption. Furthermore, because its mechanicalstrength is high, it has been desired that the glass-epoxy base materialis used as the insulator layer so as to achieve the circuit board havingthe innervia-hole structure over all the layers by the inner viaconnection.

However, when simply attempting to apply the above-describedinner-via-hole technology over all the layers to the glass-epoxy basematerial, there arises a problem in that the variations in theconnection resistance of the inner via holes increase. The inventors ofthe present invention conducted a study to find as its cause that theglass woven fabric serving as the reinforcer had variations in density(portions in which warps and wefts overlap each other and those in whichthey do not) in an in-plane direction. More specifically, in the hotpress process of heating and compression, the inner via holes providedin the low-density portion of the reinforcer (where the warps and weftsdo not overlap each other) expand laterally because there is lessreinforcement on their side wall surfaces. In other words, appliedpressure dissipates laterally. Consequently, a sufficient compressionforce is not applied in the longitudinal direction of the inner viahole, so that electrical conductors cannot be connected sufficiently,thus increasing an electrical connection resistance.

The unevenness in thickness and density in the in-plane direction causessuch variations in the electrical connection resistance not only in theglass cloth impregnated with an epoxy resin but also in non-wovenfabric, a sheet and a film.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the problems describedabove and to provide a circuit board and a method for manufacturing thesame that can achieve a high-density wiring and an inner via connectionresistance with less variation even when a base material including areinforcer sheet with density distribution in its in-plane directionsuch as a glass-epoxy base material is used for an insulator layer.

In order to achieve the above-mentioned object, a circuit board of thepresent invention includes an electrical insulator layer formed of areinforcer sheet with density distribution in its in-plane direction, anelectrical conductor filled in a plurality of inner via holes providedin the electrical insulator layer in its thickness direction, and awiring layer connected to the electrical conductor. The inner via holesprovided in a high-density portion of the reinforcer sheet are formed tohave a smaller cross-section than the inner via holes provided in alow-density portion of the reinforcer sheet.

Next, a method for manufacturing a circuit board of the presentinvention includes providing a plurality of inner via holes to be filledwith an electrically conductive paste in an insulator layer having areinforcer sheet with density distribution in its in-plane direction,with the inner via holes provided in a high-density portion of thereinforcer sheet being formed to have a smaller cross-section than theinner via holes provided in a low-density portion of the reinforcersheet, filling the electrically conductive paste in the inner via holes,and laminating a wiring layer or a metal foil for forming the wiringlayer so as to be connected to the electrically conductive paste,followed by heating and compression.

According to the present invention, it becomes possible to provide acircuit board and a method for manufacturing the same that can achieve ahigh-density wiring and an inner via connection resistance with lessvariation even when a base material including a reinforcer sheet withdensity distribution in its in-plane direction such as a glass-epoxybase material is used for an insulator layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view showing a circuit board according to afirst embodiment of the present invention.

FIGS. 2A to 2D are drawings for explaining processes in a method formanufacturing a circuit board (having through holes) according to thefirst embodiment of the present invention.

FIGS. 3A to 3B are drawings for explaining processes in the method formanufacturing the circuit board (having through holes) according to thefirst embodiment of the present invention.

FIGS. 4A to 4B are drawings for explaining processes in the method formanufacturing the circuit board (having through holes) according to thefirst embodiment of the present invention.

FIGS. 5A to 5D are drawings for explaining the method for manufacturingthe circuit board (having non-through holes) according to the firstembodiment of the present invention.

FIG. 6 is a schematic cross-sectional view showing a multilayeredcircuit board according to a second embodiment of the present invention.

FIG. 7 is a schematic cross-sectional view showing a multilayeredcircuit board according to a third embodiment of the present invention.

FIGS. 8A to 8D are drawings for explaining a method for manufacturingthe multilayered circuit board according to the third embodiment of thepresent invention.

FIGS. 9A to 9C are drawings for explaining the method for manufacturingthe multilayered circuit board according to the third embodiment of thepresent invention.

FIGS. 10A to 10B are drawings for explaining the method formanufacturing the multilayered circuit board according to the thirdembodiment of the present invention.

FIG. 11 is a schematic cross-sectional view showing a circuit boardprovided with inner via holes having protruding glass fibers accordingto the first embodiment of the present invention.

FIG. 12 is a schematic cross-sectional view showing a four-layeredcircuit board obtained in the second embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the case of making a connection by pressure using an electricallyconductive paste, the connection resistance of inner via holes suddenlybecomes unstable so as to have increased variation when the ratio of(insulator layer thickness/via hole diameter) exceeds 1. Thus, in orderto achieve a via hole with a small diameter (for example, a diameter of50 μm) in a circuit board, it is preferable that the insulator layer hasa thickness of 50 μm or smaller. However, when a glass-epoxy basematerial or an aramid-epoxy base material is used, a circuit board as acore substrate usually has a thickness of 50 μm or larger. In addition,an excessively thin core substrate is not preferable because themechanical strength is reduced. Therefore, it is preferable that a thininsulator layer has a thickness of 50 μm or smaller and the ratio of theinsulator layer thickness/the via hole diameter is 1 or smaller.

In the present invention, it is preferable that the reinforcer sheetwith density distribution in its in-plane direction is a woven fabric ora non-woven fabric formed of at least one fiber selected from the groupconsisting of a synthetic fiber and an inorganic fiber. Of course, thereinforcer sheet with density distribution in its in-plane direction maybe a film formed of a synthetic resin.

Also, it is preferable that the reinforcer sheet with densitydistribution in its in-plane direction is a woven fabric formed of aglass fiber.

Furthermore, it is preferable that the inner via holes provided inoverlapping portions of warps and wefts of the woven fabric formed ofthe glass fiber have a smaller cross-section than the inner via holesprovided in other portions.

Moreover, it is preferable that the inner via holes having a largercross-section have a smaller amount of protruding fibers on a side wallsurface thereof than the inner via holes having a smaller cross-section.

In addition, it is preferable that a plurality of the wiring layers areprovided, and at least one of the wiring layers is embedded in theinsulator layer.

It is preferable that the inner via holes have a small cross-section inthe high-density portion of the reinforcer sheet, while the inner viaholes have a large cross-section in the low-density portion of thereinforcer sheet.

Also, a circuit board formed of a compressible electrical insulatingmaterial further may be laminated on one surface of the circuit board ofthe present invention.

Furthermore, a circuit board formed of a compressible electricalinsulating material further may be laminated as a core substrate betweenthe circuit boards of the present invention arranged on both sides ofthe core substrate.

Moreover, the circuit board of the present invention is used as a coresubstrate, and at least one circuit board formed of an insulator layerthinner than the insulator layer of the core substrate further may belaminated on at least one surface of the core substrate.

In addition, the larger cross-section of the inner via holes ispreferably 1.15 to 10 times, more preferably 1.4 to 5 times,particularly preferably 1.4 to 2 times as large as the smallercross-section. The cross-section difference of less than 1.15 timesmakes it difficult to reduce variations in electrical resistance causedby the density variations of the reinforcer sheet, while that of morethan 10 times results in an excessively low via resistance, making itdifficult to reduce variations in the via resistance.

Next, in a method of the present invention, it is preferable that theinner via holes provided in the high-density portion of the reinforcersheet are formed to have a smaller cross-section than the inner viaholes provided in the low-density portion of the reinforcer sheet byinserting a rotating drill in a thickness direction of the reinforcersheet to form a through hole, stopping the drill while keeping itrotating, and then pulling out the drill.

Also, it is preferable that the inner via holes provided in thehigh-density portion of the reinforcer sheet are formed to have asmaller cross-section than the inner via holes provided in thelow-density portion of the reinforcer sheet by thermal laser machining.

Furthermore, it is preferable that a plurality of the wiring layers areprovided, and at least one of the wiring layers is embedded in theinsulator layer.

In the above description, when the glass-epoxy base material is used,the cross-section of the inner via holes in sparse portions of the glasscloth is preferably at least 1.15 times, further preferably about 1.4times as large as that of the inner via holes in the overlappingportions of warps and wefts thereof. Within this range, the variation invia resistance is reduced.

According to a circuit board of the present invention, it is possible toachieve a circuit board having connection resistance with lessvariation.

According to another circuit board of the present invention, it ispossible to achieve a circuit board having connection resistance withless variation and high connection reliability.

Also, in another circuit board of the present invention, it ispreferable that at least one of the wiring layers is embedded in theinsulator layer. With this example, it is possible to achieve a circuitboard having connection resistance with still less variation.

Furthermore, according to a multilayered circuit board of the presentinvention, it is possible to achieve a multilayered circuit board havingconnection resistance with still less variation over all the layers.

Moreover, according to a multilayered circuit board of the presentinvention, it is possible to achieve a multilayered circuit board havinga fine wiring layer on its surface by using a circuit board havingconnection resistance with less variation as a core substrate.

In addition, according to a method for manufacturing a circuit board ofthe present invention, a circuit board having connection resistance withless variation can be manufactured easily.

In the method for manufacturing a circuit board of the presentinvention, it is preferable to include embedding at least one of thewiring layers in the insulator layer. With this example, a circuit boardhaving connection resistance with still less variation can bemanufactured easily.

In addition, according to a method for manufacturing a multilayeredcircuit board of the present invention, a multilayered circuit boardhaving connection resistance with less variation over all the layers canbe manufactured easily.

Moreover, according to a method for manufacturing a multilayered circuitboard of the present invention, a multilayered circuit board having afine wiring layer on its surface can be manufactured easily by using acircuit board having connection resistance with less variation as a coresubstrate.

First, materials used in the present invention will be described.

(Electrical Conductor for Forming Inner Via Hole)

An electrical conductor used for forming inner via holes can be a resincomposition containing electrically conductive powder (an electricallyconductive paste). The electrically conductive paste is preferablebecause its electrical conductivity increases when being compressed.

An electrically conductive filler used here can be a filler formed of atleast one metal selected from the group consisting of gold, silver,copper, nickel, palladium, lead, tin, indium and bismuth, an alloythereof or a mixture thereof. It also is possible to use a coated fillerobtained by coating the above-mentioned metal or alloy onto a ballformed of the above-mentioned metal or alloy, an oxide of alumina orsilica, or an organic synthetic resin.

The form of the electrically conductive filler is not limitedspecifically, but can be powder, fibrous filler, granulated powder,spherical balls or a mixture thereof.

A resin used for a binder of the resin composition can be a liquid epoxyresin, a polyimide resin, a cyanate ester resin or a phenol resol resin.The epoxy resin can be a glycidyl ether epoxy resin such as a bisphenolA type epoxy resin, a bisphenol F type epoxy resin or a bisphenol ADtype epoxy resin, or an epoxy resin containing two or more epoxy groupssuch as an alicyclic epoxy resin, a glycidyl amine type epoxy resin or aglycidyl ester type epoxy resin. In addition, an epoxy compoundcontaining one epoxy group also may be contained as a reactive diluent.

If necessary, additives such as a dispersing agent or a solvent, forexample, butyl cellosolve, ethyl cellosolve, butyl carbitol, ethylcarbitol, butyl carbitol acetate, ethyl carbitol acetate or α-terpineolcan be present.

The electrical conductor of the present invention is not limited to theabove-mentioned electrically conductive paste, but can be an inner viaconnecting material that makes an electrical connection by pressure, forexample, a via post formed of metal such as gold, silver, copper,nickel, palladium, lead, tin, indium or bismuth.

(Electrical Insulator Layer with Density Distribution in In-planeDirection)

The material for an electrical insulator layer with density distributionin its in-plane direction can be a glass-epoxy base material. Theglass-epoxy base material is a composite material obtained byimpregnating a glass woven fabric with an epoxy resin. The glass-epoxybase materials at B stage (in a semi-cured state) and those at C stage(in a cured state) are commercially available as a material for circuitboards. They are preferable because of their excellent mechanicalstrength and availability at low cost. It is especially preferable touse the base material at B stage (in the semi-cured state) rather thanthat at C stage (in the cured state). This is because, compared with thecured state, the resin in the semi-cured state is easier to perforatewith a laser, shows larger difference in processability from the glasscloth as the reinforcer, and needs lower effective pressure forcompressing the electrically conductive paste. However, the electricalinsulator layer is not limited to these examples of the base material,but can be an insulator layer containing a reinforcer sheet with adensity distribution (density variation) in the in-plane direction. Forexample, it is possible to use a composite material sheet or a compositematerial film that is obtained by impregnating woven fabric or non-wovenfabric with a thermoplastic resin or a thermosetting resin. The wovenfabric or non-woven fabric may be formed of organic fibers such as PBO(polyparaphenylene benzobisoxazole) fibers, PBI (polybenzimidazole)fibers, aramid fibers, PTFE (polytetrafluoroethylene) fibers, PBZT(polyparaphenylene benzobisthiazole) fibers or all aromatic polyesterfibers, or inorganic fibers such as glass fibers. The thermosettingresin may be an epoxy resin, a polyimide resin, a phenolic resin, afluorocarbon resin, an unsaturated polyester resin, a PPE (polyphenyleneether) resin, a bismaleimide triazine resin or a cyanate ester resin.

The thickness of the electrical insulator layer is not limitedspecifically, but can be about 0.02 to 0.5 mm, which is a generalthickness of commercially available insulator layer. It is preferablethat the electrical insulator layer has a weight per unit area rangingfrom 50 to 800 g/m².

(Cover Film)

During the manufacturing process, a cover film serves to preventcontamination by dusts and as a mask when filling the electricalconductor, and then is removed in the end. Thus, it is preferable thatthe cover film is provided at least on the side of filling theelectrical conductor in a prepreg. It also is preferable that thesurface contacting the prepreg is subjected to a release treatment. Thematerial for the cover film is not limited specifically, but can be, forexample, one obtained by applying a silicone-based releasing agent ontoa PET (polyethylene terephthalate) film or a PEN (polyethylenenaphthalate) film. When the electrically conductive paste is filled byprinting, an excess electrically conductive paste that is as thick asthe cover film is provided above the inner via hole. After the coverfilm is peeled off lastly, the electrically conductive paste protrudesfrom the inner via holes. This protrusion corresponds to the thicknessto be compressed in a hot press process. Accordingly, as the cover filmbecomes thicker, the inner via holes are compressed more so as toachieve lower connection resistance. On the other hand, an excessivelythick cover film rips off the electrically conductive paste when beingpeeled off. For example, the thickness of the cover film preferably is35 μm or smaller when the inner via hole has a diameter of 200 μm orsmaller, while it preferably is 20 μm or smaller when the inner via holehas a diameter of 100 μm or smaller.

(Metal Foil)

A specific example of a metal foil includes an electrolytic copper foiland a rolled copper foil. In the case of the electrolytic copper foil,it is possible to use a commercially available copper foil having athickness of about 3 to 70 μm. A thin copper foil, especially one of 9μm or smaller in thickness, can be provided with a carrier for beinghandled more easily. With regard to a surface roughness of the copperfoil, an average roughness Rz ranges, for example, from 0.5 to 10 μm.

The following is a description of embodiments of the present invention,with reference to the accompanying drawings.

First Embodiment

FIG. 1 is a schematic plan view showing a circuit board of the firstembodiment. The present embodiment is directed to the case in which aglass-epoxy base material is used for an insulator layer including areinforcer sheet 101 with density distribution in its in-planedirection. To facilitate an explanation, FIG. 1 shows wefts 102 a andwarps 102 b of a glass woven fabric inside the base material. An innervia hole 103 that is provided in a portion other than an overlappingportion of glass fibers (a high-density portion of the reinforcer sheet)has a larger cross-section than an inner via hole 104 provided in theoverlapping portion. In this embodiment, the cross-section of the innervia hole 103 in the sparse portion of the glass cloth is preferably atleast 1.15 times, more preferably at least 1.4 times as large as that ofthe inner via hole 104 in the overlapping portion thereof. Within thisrange, the variation in via resistance is reduced.

A circuit board of the present embodiment can be produced as follows.

First, a connection intermediate is produced. Cover films 202 areprovided by thermocompression bonding on both surfaces of a glass-epoxybase material (a glass-epoxy prepreg 201) at B stage (in a semi-curedstate). Then, inner via holes (through holes 203 and 203′ in the presentembodiment) are formed at desired positions by a mechanical drill (seeFIG. 2A).

Next, the inner via holes are filled with an electrically conductivepaste 204 by printing, and then the cover films 202 are peeled off, thuscompleting a connection intermediate 205 (see FIG. 2B).

As one exemplary condition of the perforating process in FIG. 2A, thethrough holes 203 and 203′ are formed at a processing speed of about 133holes per minute by a drill having a diameter of 150 μm and a loweringspeed of 2 m per minute. After the hole is pierced, the drill is stoppedat a lowered position while being kept rotating for about 0.2 seconds,for example, and then pulled out.

In this case, the diameter of the hole 203 in the dense portion remainsintact because the fibers serve as braces, while that of the hole 203′in the sparse portion (the portion rich in resin) increases owing toheat generated during processing, slight deflections of the drill centeror the like. In other words, the hole diameter varies continuouslyaccording to a fiber amount of a processed portion, so that the holediameter is inversely proportional to the fiber density in the portionto be perforated. The term “inversely proportional” does not refer tobeing inversely proportional in a mathematical sense, but means that“the hole diameter is small in the portion with an increased reinforcerdensity, while the hole diameter is large in the portion with adecreased reinforcer density.” Hereinafter, this term will be used inthis sense.

In a usual production process of the circuit board, the drill normallyis lifted off immediately after penetrating a material. This is forensuring hole quality (forming holes with a uniform diameter),preventing the drill breakage and improving efficiency. In this case,holes having substantially the same diameter are formed regardless ofthe density of the fibers.

For example, when holes were formed according to the present embodimentby using a prepreg having a thickness of about 70 μm, the holes in theoverlapping portions of the warps and wefts of the glass cloth (thehighest-density portions of the glass fibers) had a diameter of 150 μm,those in the sparse portions of the cloth (the lowest-density portionsof the glass fibers) had a diameter of 180 μm, and those in the rest hada diameter varying from 150 to 180 μm inversely with the glass fiberdensity. The holes provided in the overlapping portions of the warps andwefts of the glass cloth were small enough to fit in the overlappingportions.

Next, metal foils 206 having a thickness of 18 μm for forming a wiringpattern are superposed on both surfaces of the connection intermediate205, followed by heating and compression with a hot press (see FIG. 2C).The condition of the hot press can be the one for general circuit boardsand, for example, at 180° C. to 250° C. at 30 to 200 kgf/cm² for 0.5 to2 hours. This process cures the resin in the prepreg and the resin inthe electrically conductive paste, thus adhering the metal foils to theprepreg and electrically connecting the metal foils on both sides viathe electrically conductive paste.

Finally, the metal foil is processed into a wiring pattern 207, thuscompleting a double-sided circuit board 208 (see FIG. 2D). The wiringpattern can be processed by a general wiring processing technique forcircuit boards such as photolithography.

The connection resistance of the inner via hole decreases withincreasing cross-section of the inner via hole. Also, as an effectivepressure applied to electrically conductive fillers (an electricallyconductive filler and a copper foil) becomes larger, the number and thesize of contacts increase, thus reducing the connection resistance. Inorder to increase the effective pressure, it is necessary not only toraise the pressure of the hot press but also to adopt a structure inwhich a side wall surface of the inner via hole does not expandlaterally. In this embodiment, such a structure can be achieved byproviding the holes that are small enough to fit in the overlappingportions of the glass cloth.

In the present embodiment, the inner via holes (through holes) areformed to have a diameter of 150 μm in the highest-density portions ofthe reinforcer (the overlapping portions of the fibers of the glasscloth) because the side wall surfaces of these portions do not expandeasily. The inner via holes are formed to have a diameter of 180 μm inthe lowest-density portions of the reinforcer (the sparse portions ofthe glass cloth) because the effective pressure can be applied leasteasily. In the other portions, the inner via holes are formed to have adiameter varying from 150 to 180 μm inversely with the glass fiberdensity. The inner via holes of the present embodiment produced underthe above-mentioned perforating condition can achieve a connectionresistance with very small variation of about 2 to 3 mΩ. However, thediameter of the inner via holes is not limited to those described above.

The above-described connection intermediates 205 and the metal foils 206are superposed on both sides of the double-sided circuit board 208 ofthe present embodiment as a core substrate (see FIG. 3A), and the coresubstrate and the prepregs are laminated with the hot press as in thecase of the double-sided circuit board. Finally, the metal foils areprocessed into a wiring pattern 209, thus obtaining a four-layeredcircuit board (see FIG. 3B).

Further, a multilayered circuit board can be produced by repeating theabove-described laminating process using the multilayered circuit boardas a core substrate.

In the multilayered circuit board of the present embodiment, the wiringlayers 207 of the core substrate are embedded in the prepregs to belaminated on both sides of the core substrate. In other words, since thewiring layers are embedded also in the inner via portions, thecompressibility of the inner via holes increases during the pressprocess, thereby further lowering the connection resistance and reducingthe variations thereof.

Also in the double-sided circuit board, the wiring layers can beembedded by using a wiring transferring method, thereby achieving astill lower connection resistance with reduced variations in a similarmanner. More specifically, as shown in FIG. 4A, it is possible to usewhat is called a metal foil provided with a carrier, which is obtainedby forming a metal foil on a support substrate (a carrier). An exampleof the metal foil provided with the carrier 211 includes commerciallyavailable metal foils that are obtained by laminating a copper foil ontoan aluminum carrier via a releasing layer. In the case of the embodimentof the present invention, the copper foil is patterned by etching with aferric chloride solution or an ammonium persulfate solution beforehand,and then the wiring layers 210 are laminated so as to be embedded in theconnection intermediate 205. Subsequently, the aluminum carrier can beremoved by etching with hydrochloric acid or the like (see FIG. 4B).

The method for forming through holes using the mechanical drill in thepresent embodiment, of course, is not limited to the above but can bethe one using drills having different diameters. In other words, theinner via holes in the portions other than the overlapping portions ofthe glass fibers (the high-density portion of the reinforcer sheet) areformed by a drill having a smaller diameter than that used for formingthe inner via holes in the overlapping portions. The drill diameter hasto be selected according to density distribution in every working whenthe density distribution of the reinforcer is irregular. On the otherhand, it is preferable to use the reinforcer with regular densitydistribution such as a glass woven fabric because such an extra processis not needed (or is reduced, thus the process becomes simple).

Other than the above method for forming the through holes, the inner viaholes (thorough holes) of the present embodiment similarly can be formedby a regular perforating method of circuit boards, that is, by a carbondioxide gas laser, a YAG laser or an excimer laser, or by punching.

As shown in FIG. 11, when the through holes are formed by the carbondioxide gas laser, inner via holes 702 formed in the high-densityportions of the glass fibers (inner via holes having a small diameter)have many glass fibers 704 protruding toward the inside of the inner viaholes. On the other hand, inner via holes 701 formed in the low-densityportions of the glass fibers (inner via holes having a large diameter)have relatively fewer glass fibers 703 protruding toward the inside ofthe inner via holes. With such a structure, the inner via holes and theglass-epoxy base material of the surrounding insulator layer adhere wellto each other by an anchor effect, thus increasing a strength withrespect to a mechanical (or a thermal) stress. This raises theconnection reliability of the inner via holes having a small diameter.Since the inner via holes having a small diameter have fewer contactswith the electrical conductors, the connection reliability thereof islikely to become lower than that of the inner via holes having a largediameter. However, the connection reliability of the inner via holeshaving a small diameter can be raised by the above method, making itpossible to improve the connection reliability of the entire substrate.

In the case of using a carbon dioxide gas laser, for example, a carbondioxide gas laser with a wavelength of 9.4 or 10.6 μm can be used. Thenumber of shots suitably is 1 to 3. In this example, the effect of thepresent invention became greater as wavelength increased and the numberof shots decreased. The perforating process utilizes the difference inprocessability that is caused by the difference of the densitydistribution of the glass cloth base material when irradiating the samelaser. This relationship is shown below.

Density Glass cloth Perforating processability Large Warps and weftsoverlap Not easy Small Sparsely woven Easy

Next, the wavelength of the laser beam will be described. In a laserhaving the same energy, a decrease in wavelength generally reduces alaser spot diameter, and thus increases an energy density of the laser.The laser having a large energy density makes it easier to perforate asheet using a glass cloth, which generally is difficult to perforate,thus forming holes having a diameter with less variation regardless ofthe density distribution of the base material. On the other hand, anincrease in wavelength expands a laser spot diameter, and thus reducesthe energy density. Accordingly, the portion of a matrix resin, which iseasy to perforate, can be perforated easily, while the portion of theglass cloth is difficult to perforate, so as to be susceptible to thedensity distribution of the base material. In other words, theoverlapping portions of the warps and wefts of the glass cloth areprovided with small holes, while the sparse portions are provided withlarge holes. Therefore, it is preferable that the wavelength of thelaser beam is large.

In the following, the number of shots will be described. In the laserirradiation, an increase in the number of shots raises the entire amountof energy input. For example, two shots require twice as much energy asone shot. Thus, a plurality of shots at one position raises the entireamount of energy, so that the glass cloth, which could not be processedby the first shot, can be perforated by the second or the third shot,thereby achieving a uniform diameter regardless of the densitydistribution of the base material. On the other hand, the small numberof shots raises the susceptibility to the density distribution of thebase material. In other words, the overlapping portions of the warps andwefts of the glass cloth are provided with small holes, while the sparseportions are provided with large holes. Therefore, it is preferable thatthe number of the shots is 1 to 3.

In the present embodiment, the inner via holes are through holes, butthey may be non-through holes. The method for producing the circuitboard in the case of the non-through holes is illustrated in FIGS. 5A to5D.

First, a wiring transferring material provided with a wiring pattern 302is placed on one surface of a prepreg 301 so that its wiring faces theprepreg, while a cover film 304 is placed on the other surface thereof,and then they temporarily are attached by pressure. Next, blind viaholes (non-through holes) 305 are formed at desired positions by acarbon dioxide gas laser or the like (see FIG. 5A), and then filled withan electrical conductor (an electrically conductive paste) 306.Subsequently, the cover film is removed (see FIG. 5B), and a metal foil307 is superposed on the side where the cover film has been removed,followed by heating and compression by a hot press (see FIG. 5C). Themetal foil is processed into a wiring pattern 308, and then a supportsubstrate 303 of the wiring transferring material is removed, thuscompleting a double-sided circuit board (see FIG. 5D). When using alayered product (a circuit board transferring material), whose metalfoil has been processed into the wiring pattern, as a transferringmaterial and repeating the above-described process necessary times, itis possible to produce a multilayered circuit board. With this method,since via holes are formed at positions according to the position of thewiring pattern, it is possible to improve the dimensional accuracy.

Second Embodiment

FIG. 6 is a schematic cross-sectional view showing a multilayeredcircuit board according to the second embodiment of the presentinvention. The multilayered circuit board of the present embodiment hasa structure in which the circuit board described in the first embodimentis laminated on at least one surface of a core substrate formed of acompressible insulator base material. In this figure, a double-sidedcircuit board 401 of an aramid-epoxy substrate is used as the coresubstrate, and circuit boards 402 formed of a glass-epoxy base materialdescribed in the first embodiment are laminated on both sides of thecore substrate.

The multilayered circuit board of the present embodiment can be producedas follows.

First, a double-sided circuit board is produced by using an aramid-epoxyprepreg. Cover films temporarily are attached by pressure onto bothsurfaces of the aramid-epoxy prepreg, and then through holes are formed.The through holes can be formed to have a diameter of 200 μm by, forexample, a carbon dioxide gas laser. The aramid-epoxy prepreg is acomposite material obtained by impregnating a non-woven fabric of aramidfibers with an epoxy resin. Since the aramid-epoxy prepreg has manypores therein so as to be compressible, inner via holes can achieveconnection reliability with less variation even without using the methoddescribed in the first embodiment. Needless to say, it is preferable tochange the hole diameter according to the density of the reinforcer (thearamid non-woven fabric, in this case) so as to achieve still lessvariation, as shown in the first embodiment.

Next, the through holes are filled with an electrically conductivepaste, and the cover film is removed, thus completing a connectionintermediate formed of the aramid-epoxy base material. Thereafter, adouble-sided circuit board of the aramid-epoxy substrate can be obtainedas in the first embodiment. In addition, the through holes may be formedusing a laser or a drill.

Using this as the core substrate, the connection intermediates and themetal foils described in the first embodiment are superposed on bothsides of the core substrate, laminated by a hot press as in the firstembodiment, and then the metal foils are processed into a wiringpattern. Thus, a four-layered circuit board (four-layer here indicatesfour wiring layers) is completed. The compressible core substrate may bea multilayered circuit board. FIG. 12 shows an example of the fourwiring layers as the core substrate.

If necessary, a multilayered circuit board with still more layers can beproduced by using the multilayered circuit board of the presentembodiment as the core substrate and repeating the process of thepresent embodiment.

In the multilayered circuit board of the present embodiment, the wiringlayers are embedded in all the insulator layers of the glass-epoxy basematerial. Thus, as described in the first embodiment, it is possible toachieve connection resistance with still less variation. Although thewiring layer is not embedded in the core substrate, the compressibilityof the aramid-epoxy prepreg compensates for this, so that the inner viaholes can be compressed sufficiently. In other words, the multilayeredcircuit board of the present embodiment can achieve connectionresistance with still less variation over all the layers. Moreover, whenthe glass-epoxy base materials are laminated on both sides, it becomesless likely that the aramid-epoxy base material is exposed and absorbsmoisture. Furthermore, since the glass-epoxy base material has anexcellent mechanical strength, it is possible to achieve a substratehaving a better mechanical strength than the multilayered circuit boardformed by the aramid-epoxy base material alone.

Third Embodiment

FIG. 7 is a schematic cross-sectional view showing a multilayeredcircuit board according to the third embodiment of the presentinvention. The multilayered circuit board according to the presentembodiment has a structure in which a circuit board 501 of the first orthe second embodiment is used as a core substrate and a circuit board502 with an insulator layer thinner than that of the core substrate islaminated on at least one surface of the core substrate. In a thinnerinsulator layer, it is possible to form finer inner via holes with lowerresistance. This is because, even when the hole diameter is the same, areduction in the length of the inner via hole, namely, the thickness ofthe insulator layer, decreases the connection resistance.

The following is a description of the case of using the four-layeredcircuit board of the second embodiment as the core substrate and apolyimide film as the insulator layer thinner than that of the coresubstrate.

The multilayered circuit board of the present embodiment can be producedas follows. First, the method for producing a substrate of the insulatorlayer of the polyimide film will be described. A cover film 604 isplaced on one surface of a film (a thin insulator layer 603) obtained byforming adhesive layers 602 on both surfaces of a polyimide film 601 asshown in FIG. 8A, and a wiring pattern provided with a carrier 605 isplaced on the other surface thereof, and then they temporarily areattached by pressure as shown in FIG. 8B. The adhesive layers 602 can bea polyimide-based adhesive or an epoxy-based adhesive. With regard tothe thickness of the film, adhesive layers of 5 μm thickness each areformed on both surfaces of a polyimide film of 13 μm thickness, forexample. The cover film can be the same as that of the first embodiment.Also, the wiring pattern can be the one obtained by forming a wiringpattern on a copper foil provided with a carrier used for a transferringmethod, which is described in the first embodiment.

Next, as shown in FIG. 8C, non-through holes are formed in the film andfilled with an electrical conductor 606, and then the cover film isremoved. Thus, a double-sided circuit transferring material intermediate614 is completed. The non-through holes can be formed by a laserperforating method. For example, an UV-YAG laser (the third harmonic:wavelength of 355 nm) can be used. The UV-YAG laser is preferablebecause it is possible to form fine non-through holes (having a diameterof about 30 to 50 μm in the present embodiment) without damaging thecopper foil.

The electrical conductor can be an electrically conductive paste as inthe first embodiment. The electrically conductive paste can be filled byprinting using a squeegee. It is preferable that pressure is reducedduring or after the filling of the non-through holes. This pressurereduction is for removing voids taken in when filling the paste from anopening. Also, it is preferable that a roughened copper foil whosesurface is provided with roughness is used as the copper foil so as toattach temporarily onto the adhesive layer while leaving a space (a finespace corresponding to the roughness of the copper foil surface andbeing smaller than an electrically conductive filler of the electricallyconductive paste) therebetween. This is because the resin contained inthe electrically conductive paste escapes from this space during thefilling of the electrically conductive paste or the compression, so thatthe ratio of an electrically conductive powder contained in the innervia hole increases, thereby achieving still lower resistance.

Then, as shown in FIG. 8D, a metal foil 607 is superposed on thedouble-sided circuit transferring material intermediate 614 on the sidewhere the cover film has been removed, followed by heating andcompression by a hot press. At this time, the wiring pattern is embeddedinto the adhesive layer 602. The condition of the hot press can be thesame as that in the first embodiment.

Subsequently, the metal foil is processed into a wiring pattern 608 by anormal photolithography, thus completing a double-sided circuittransferring material provided with a carrier 609 (see FIG. 9A).

When using the double-sided circuit transferring material provided withthe carrier 609 as the metal foil provided with the carrier andrepeating the above-described process, it is possible to produce amultilayered transferring material intermediate 610 (see FIG. 9B) and amultilayered circuit transferring material 611 (see FIG. 9C).

The above description is directed to the case of using the polyimidefilm as the insulator layer thinner than that of the core substrate.However, it also is possible to use the one obtained by forming anadhesive on a film formed of a material such as BCB (benzocyclobutene),PTFE (polytetrafluoroethylene), aramid, PBO (polyparaphenylenebenzobisoxazole) or all aromatic polyester. When using a thermoplasticfilm, the film can be used without an adhesive because the film itselfbecomes adhesive when being heated.

Next, the transferring material is laminated on the core substrate.

The core substrate can be a circuit board 612 described in the first orthe second embodiment. The transferring material intermediate 610 issuperposed on at least one surface of the core substrate 612 as shown inFIG. 10A, and then laminated by a hot press. The condition of the hotpress can be the same as that in the first embodiment. Finally, thecarrier of the transferring material intermediate is removed by etching,thus completing a multilayered circuit board of the present embodiment.

Also, instead of using the transferring material intermediate, a(multilayered) circuit transferring material 611 of the presentembodiment is laminated on the core substrate 612 via a connectionintermediate 613 of the first or the second embodiment, therebyproducing the multilayered circuit board of the present embodiment (seeFIG 10B).

The present embodiment is directed to the method of transferring thethin insulator layer on the core substrate with the wiring transferringmaterial. With this method, the finer circuit formed on the thininsulator layer and the core substrate can be produced separately. As aresult, it becomes possible to reduce contamination by dust in the finecircuit portion and improve an overall yield compared with the method ofsequentially laminating layers on top of the core substrate.

In the multilayered circuit board of the present embodiment, it ispossible to use the circuit board having an IVH structure over all thelayers provided with connection resistance with less variation, which isdescribed in the first and second embodiments, as a core substrate so asto form wiring layers with higher density thereon. The circuit boardproduced with an insulator layer of a thin polyimide film alone isdifficult to be applied to a field requiring a mechanical strength. Onthe other hand, the multilayered circuit board of the present embodimentcan achieve the mechanical strength and the high-density fine wiring(including the core substrate) and especially is preferable as a circuitboard for package on which a relatively large semiconductor is mounteddirectly.

The core substrate can be a general circuit board (a glass-epoxy throughhole circuit board, a built-up circuit board or a multilayered circuitboard using an aramid fiber non-woven fabric impregnated with an epoxyresin). In addition, as shown in the figure, the above-describedtransferring material can be laminated directly on the metal foil forforming the wiring via the connection intermediate.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The embodimentsdisclosed in this application are to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, all changes that come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

What is claimed is:
 1. A circuit board comprising: an electricalinsulator layer formed of a reinforcer sheet with density distributionin its in-plane direction; an electrical conductor filled in a pluralityof inner via holes provided in the electrical insulator layer in itsthickness direction; and a wiring layer connected to the electricalconductor; wherein the inner via holes provided in a high-densityportion of the reinforcer sheet are formed to have a smallercross-section than the inner via holes provided in a low-density portionof the reinforcer sheet, and wherein the larger cross-section of theinner via holes is 1.15 to 10 times as large as the smallercross-section.
 2. The circuit board according to claim 1, wherein thereinforcer sheet with density distribution in its in-plane direction isat least one sheet selected from the group consisting of a woven fabricand a non-woven fabric formed of at least one fiber selected from thegroup consisting of a synthetic fiber and an inorganic fiber.
 3. Thecircuit board according to claim 1, wherein the reinforcer sheet withdensity distribution in its in-plane direction is a film formed of asynthetic resin.
 4. The circuit board according to claim 2, wherein thereinforcer sheet with density distribution in its in-plane direction isthe woven fabric formed of a glass fiber.
 5. The circuit board accordingto claim 4, wherein the inner via holes provided in overlapping portionsof warps and wefts of the woven fabric formed of the glass fiber have asmaller cross-section than the inner via holes provided in portionsother than the overlapping portions.
 6. The circuit board according toclaim 1, wherein the inner via holes having a larger cross-section havea smaller amount of protruding fibers on a side wall surface thereofthan the inner via holes having a smaller cross-section.
 7. The circuitboard according to claim 1, wherein a plurality of the wiring layers areprovided, and at least one of the wiring layers is embedded in theinsulator layer.
 8. The circuit board according to claim 1, wherein thecross-section of the inner via holes is tapered to vary continuously,and the inner via holes have a small cross-section in the high-densityportion of the reinforcer sheet, while the inner via holes have a largecross-section in the low-density portion of the reinforcer sheet.
 9. Thecircuit board according to claim 1, wherein a circuit board formed of acompressible electrical insulating material further is laminated on onesurface of the circuit board of claim
 1. 10. A circuit board assembly,comprising a circuit board formed of a compressible electricalinsulating material as a core substrate, and a circuit board of claim 1laminated on each side of the core substrate.
 11. A circuit boardassembly, comprising a circuit board of claim 1 as a core substrate, andat least one circuit board formed of an insulator layer thinner than theinsulator layer of the core substrate further is laminated on at leastone surface of the core substrate.